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    Re: Historical question
    From: Alexandre Eremenko
    Date: 2004 Oct 23, 19:04 -0500

    I have a conjecture explaining the misterious paragraph of Norie.
    Before I state it, let me continue that citation a bit:
    
    "...having a good Quadrant to take the altitudes,
    and a Sextant to observe the distances.
    Let the observations be taken in the following order,
    noting the times by a watch: 1, the altitude of the Sun
    or Star; 2, the altitude of the Moon; 3. any number of distances;
    4, the altitude of the Moon; 5, the altitude of the Sun or Star".
    
    First, let us assume that a navigator who could afford a
    good sextant (worth about 20 pounds sterling
    at the time when
    gold was 4 pounds 8s per ounce).
    would also have a wooden quadrant, as a backup, or to permit
    his assistants to handle. (Its price was about 1/4 of that
    of a sextant if I remember correctly). So the presence
    of the second
    instrument on board is not surprising.
    
    Second, we have to take into account that
    a) The reading of those sextants was much slower than of
    the modern ones. They had a long Nonius (Vernier), permitting
    to read to 10", and the Nonius had to be read through a microscope.
    b) It takes some time to catch both bodies (or a body
    and the horizon)
    in
    the filed of view.
    
    Then using both instruments (if you already have them!)
    for this sort of observation makes a sense. All distance
    observations, made by the Sextant, with maximal precision
    are not interrupted by altitude observations.
    And the quadrant (which was already set for altitude observations
    does not have to be reset.
    
    This permits the series of observations in a quick sequence.
    Using the quadrant in addition to the sextant indeed saves some
    time.
    
    Notice, that the whole procedure is designed to minimize
    the total time of the observation (see the discussion on Averaging).
    
    Now let me continue the citation a bit further:
    
    "Now add together the distances, and the times when they are taken,
    each of which being divided by the number observed...."
    
    Alex.
    
    
    

       
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